The present invention relates to a secondary battery (storage battery) which can be charged and repeatedly used. More particularly, the invention relates to a negative electrode structure of the secondary battery.
In these days, electric and electronic devices having a light and thin structure have been developed and thus a thin secondary battery having a light and compact structure is required to be used as a battery for these compact electric and electronic devices. For this purpose, a polymer battery has been considered and proposed to be used as such a battery. The polymer battery comprises an active material of an electrode made from a polymer material such as polypyrrole, polyaniline, polyparaphenylene, polyacetylene, etc. which polymer material is adequate for producing a light and thin battery having a compact size. Examples of such a polymer battery are disclosed in British Patent No. 216549 and U.S. Pat. No. 4,442,187. From the above-mentioned polymer material, it is possible to electrochemically form a P-type conductive or semiconductive polymer or an N-type conductive or semiconductive polymer by combining the above-mentioned polymer material with an electrolyte cation (positive ion) or anion (negative ion) to form a complex material therefrom.
This reaction can be electrochemically repeated. Therefore, the above-mentioned polymer materials can be used as an electrode material for the secondary battery which is repeatedly used by charging the battery after use (discharge). The energy density per weight of each of these polymer materials is very high when compared to conventional electrode materials, so that a secondary battery having a high energy ratio is expected to be obtained by using one of the above-mentioned polymer materials as the electrode material thereof.
An experimental result shows that the energy density of the secondary battery having a positive electrode comprising the above-mentioned polymer material is such high as 80 to 160 Ah/kg. However, no sufficient installation technique has been developed so far which technique is properly applied for actually utilizing the polymer material as the electrode of the secondary battery so as to heighten the energy density thereof. Also, the polymer material itself has not a high electric conductivity sufficient enough for use as the battery electrode. Further, the diffusibility of ion of the polymer material is limited Therefore, widening the surface area of the electrode is considered as one of effective means for utilizing the polymer material to obtain a high energy output from the battery. Especially, when used as a positive electrode of the battery, the polymer material is advantageous for heightening the energy output since the polymer material is more flexible than the inorganic materials and can be formed as a sheet or film. The electrode made from the sheet material can be rolled to form a cylindrical shape and installed in the battery (cylindrical type cell) in the same manner as the Ni-Cd battery.
The inventors of the present invention proposed, in Japanese Patent Application No. 62-168280, a sandwich type cell in which a positive electrode sheet and a negative electrode sheet are folded alternately through a separator to obtain high energy and high output from the battery. However, the negative electrode sheet which can be adequately utilized to be assembled in the battery as proposed has not been realized so far. With respect to the positive polymer electrode, a high energy density can be realized when the polymer is combined with an alkali metal to cooperate with the metal in a non-aqueous electrolytic solution. The negative electrode for this positive electrode is made from for example lithium In the secondary battery comprising the lithium negative electrode, lithium repeats dissolution and deposition during the charging process and the discharging process. In this electrochemical process, lithium cation added to the solution as an electrolytic salt is enough for repeating the process. In accordance with this theory, it has been proposed to use a metal sheet comprising no active material as the negative electrode in which the active material is compensated for by the cation dissolved in the electrolytic solution. However, with this proposed arrangement, it is difficult to maintain a predetermined sufficient discharge voltage. Also, if the active material is to be compensated for by excess charge, the positive polymer electrode will be damaged.
On the other hand, lithium can be formed as a flexible sheet of thickness less than 100 .mu.m. Therefore, it is most desirable if lithium can be utilized as the material of the negative electrode. However, thickness of the lithium sheet becomes uneven so that the sheet loses the shape thereof as a sheet by the repetition of dissolution and deposition of lithium during charging and discharging the battery. Also, dendrites are generated from the lithium sheet electrode, which causes a short circuit between the positive electrode and the negative electrode. In order to solve these problems, it has been proposed to alloy lithium so as to prevent lithium from dissolution and avoid dendrite generation. However, it is very difficult to form a sheet from lithium alloy. For example, lithium alloy combined with aluminum is too fragile to form a sheet therefrom.
Flexible negative electrode can not be obtained from the Li-Al alloy if a layer of the Li-Al alloy is not thin in thickness thereof. However, sufficient characteristic of a charge and discharge cycle can not be provided for the secondary battery when the layer of the Li-Al alloy is thin in thickness thereof. Therefore, it is necessary to make thick the thickness of the Li-Al layer in order to ensure the sufficient characteristic of the charge and discharge cycle. Also, as another method for utilizing lithium or lithium alloy, it has been proposed to laminate the metal with a foil of nickel or stainless steel by for example pressure welding.
However, the thickness of the negative electrode made from the laminated sheet becomes more than 100 .mu.m, which is disadvantageous for realizing a thin secondary battery. Further, as a still another method for utilizing lithium or lithium alloy, it has been proposed to form a lithium alloy layer on an aluminum sheet surface by an electrolytic process using the aluminum sheet as a substrate. However, a desired electrode sheet has not been realized by this method since the fresh lithium deposited by the electrolytic process is very active and readily reacts with the solvent.